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Fan location for DIY LED fixture.


disaster999

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disaster999

Im upgrading my 30" LED fixture to a 54" long for me tank. I have always planned for adding a shroud on my light to keep everything nice and tidy but I never actually gotten around in doing it. Before i have 2 Fans on my fixture 1/3 from the edge of the fixture blowing onto the heatsink and the hot air will just flow to where ever

 

This time around, I want to do it right and will put a shroud on my light to make it look nice and pretty. My problem now is where should i put the fans. If I go with option 1, I will have a dead space in the center and wont get adequate cooling unless I put slots near the center to allow the hot air to vent. But I want to avoid that as i dont want to ruin the look im after.

 

Option 2 and 3 would seem like a possible solution but I wonder if this is the best. Can anyone suggest any other alternative solution?

 

 

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disaster999

I had 5 clusters of 15 led on my light and 2 92mm fans was able to keep the heatsink at around 40c. well within the operating range of the LED.

 

My new setup I will have 7 clusters, 2 of them will be the 70W Lumia chip from LEDgroupBuy and an extra 2 ft of heatsink. I dont expect the temps to be significantly higher than my current setup.

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Option 1 is ideal, as it better distributes the airflow over the length of the heatsink, but without venting in the center, you create a slight problem with the airflow from each fan fighting each other. Really though, the volume of the plenum is probably large enough that the air blown into the center will find it's way back out without much difficulty.

 

Out of the other two, go with option 2. You want to create positive pressure inside the plenum. It far more effective than trying to suck the air in from the ends and creating a negative pressure.

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disaster999

Thanks Evil. Option1 it is. I think I can come up with something to vent the exhaust out in the center without disrupting the look.

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Add a third fan?

 

Maybe overkill, but if you put it in the centre and had it extracting instead of blowing it would draw air from the 2 fans 1/3rd of the way in from each end preventing backpressure.

Alternatively some simple slots or a cut out with a fine alloy mesh or something would indeed work just as well, could always make the cut out something cool like the shape of a fish or something.

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my fixture is something like your option 3 and works fine. Just be sure that the fixture is sealed enough that the air comes really from the sides and not from a hole next to fan. Also make the inlet holes big enough (depend by your fan flow) or you may hear a whistle :). Look on your computer - all fans suck air from the inside to outside....

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Computer cases are a little different, you want general air movement in there to provide cooling for all components. However CPU and graphic cards in more powerful machines have their own independent fans generally and they are always blowing onto a heat sink connected directly to the component to get maximum air movement over the cooling surface. The case extractor fan is there to remove the hot air coming off these components so they do not over heat as the ambient air temperature in the case increases.

As you say if you seal your heat sink in well enough fans will be able to draw air over the fins, but as Evil has said it is far more efficient to blow air onto a heat sink. The fans also need to be resting directly on the fins, my first LED build I had the fans 10mm off the heat sinks on the unit cover and the heat sink got incredibly hot, as soon as the fans were rested on the heat sink it never got more than warm. The same goes for if you intend to suck air over the heat sink, you need a cover resting on the fins so air is drawn between them, not just from over them.

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disaster999

Thanks for the info. Ill go with Option 1 and have it blow directly on the heat sink. Ill make sure the Fans are directly on the heat sink.

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